Measuring means for the bath resistance of aluminum reduction cells



Dec. 22, 1959 LUNDBORG 2,918,421

C. J. MEASURING MEANS FOR THE BATH RESISTANCE OF ALUMINUM REDUCTIONCELLS Filed June 14. 1957 K24 INVENTOR CARL J. LUN DBORG ATTORNEYS (M.ma

United States Patent MEASURING MEANS FOR THE BATH RESIST- AN CE OFALUMINUM REDUCTION CELLS Carl J. Lundborg, Butte, Mont, assignor toAnaconda Aluminum Company, New York, N.Y., a corporation of Montana Thisinvention relates to electrical apparatus for measuring the resistancebetween the cathode and movable anode of an aluminum reductionelectrolytic cell, to provide a guide for adjusting the movable anode toits optimum position with respect to the cathode. In a preferredembodiment, the invention provides means to measure the resistance, andhence to indicate the optimum anode adjustment, in any one of a seriesof aluininum reduction cells. In general these results are achieved bythe provision of electrical circuits which compensate for those portionsof the total voltage drop through the cell due to fixed resistances ofthe cell and its power supply circuit and due to the electrochemicalreaction taking place in the cell, and which utilize the voltage aftersuch compensations have been made to indicate the resistance of theelectrolytic bath between the anode and cathode.

Aluminum metal is conventionally produced by the electrolyticdecomposition of alumina in a molten bath of alumina andcryolitecontained in an electrolytic reduction cell (or pot, as it isusually called). In such a process, current is passed through the bathbetween a conductive carbon lining serving as the cathode of the celland a conductive carbon anode partially immersed in the bath. Aluminummetal deposits at the cathode and oxygen is released at the anode. Atthe high temperature prevailing, the oxygen combines with the carbon ofthe anode. The anode therefore is mounted on jacks so that it may belowered into the bath as it is consumed by oxidation, thus permitting asubstantially constant distance to be maintained between the anode andcathode. In this manner the bath resistance to the flow of currentthrough the cell, and the amount of current which flows through thecell, are both held at substantially constant .values by continuousadjustment of the jacks. It is the primary object of this invention toprovide means for gauging the required adjustment of the anode byindicating the value of the bath resistance.

The total voltage applied across the terminals of an aluminum reductioncell must equal the decomposition potential of aluminua plus the sum ofthe voltage drops due to the fixed resistances of the cell and its powerconnections and to the somewhat variable resistance of the electrolyticbath between the anode and the cathode. The fixed resistances of a givencell are readily measurable, and the decomposition potential of aluminain a molten cryolite solution is known to be substantially equal to 1.7volts. The bath resistance, however, tends to increase as the bottomface of the anode burns away and widens the spacing between anode andcathode, and in consequence the current through the cell tends todiminish. It is therefore necessary to adjust the position of the anode,either continuously or from time to time, to maintain the bathresistance substantially constant at an optimum predetermined value.

Heretofore it has been common practice to adjust the position of theanode with respect to the cathode to ice secure a predetermined over-allvoltage drop through the cell. This voltage drop is a function of thecurrent flowing through the cell, and such current-may (and frequentlydoes) change during adjustment of the anode position. it is thereforepreferable to make the anode adjustment to maintain a predeterminedvalue of bath resistance. The present invention provides means fordetermining the bath resistance at any moment and gauging the anodeadjustment accordingly.

The bath resistance measuring means of the invention comprises aproportional current circuit including a potentiometer resistanceelement, an adjustable resistance and a source of direct currentconnected in series. This circuit further includes means for maintainingthe current flowing in it in fixed proportion to the current flowingthrough the electrolytic cell. Other circuit means are provided forapplying a residual voltage, equal to the total electrolytic cellvoltage minus the decomposition potential of alumina, across theadjustable resistance plus a variable portion of the potentiometerresistance in said proportional current circuit; and means are providedfor varying the variable portion of the potentiometer resistance to apoint where the voltage drop due to current flowing through saidvariable portion and said adjustable resistance equals the residualvoltage appiied thereto. At the point of balance, the magnitude of thevariable portion of the potentiometer resistance is proportional to theresistance of the electrolytic bath, provided the adjustable resistancehas been set at a value such that the voltage drop across it due tocurrent flowing through it is equal to the voltage drop due to currentflowing through the fixed resistances of the electrolytic cell. v

The circuit means for applying the correct residual voltage to theadjustable resistance and the variable portion of the potentiometerresistance in. the proportional current circuit preferably comprises acircuit connecting the terminals of the electrolytic cell, in serieswith a source of back E.M.F. equal to the decomposition of voltagealumina, across said resistances. Advantageously a currentsensing meansis connected in series with the source of back E.M.F., and an electricmotor is provided to adjust the potentiometer to a position at which thecurrent-sensing means gives a null indication. The current-sensing meansin turn may serve to actuate the circuit through which the motor isenergized, so as to drive the motor in the direction necessary toproduce a null indication in the current-sensing means.

The means for maintaining the current in the proportional currentcircuit in fixed proportion to the current flowing through theelectrolytic cell advantageously comprises a shunt resistor connectedeffectively in parallel with a shunt in the electrolytic cell circuitand therefore normally carrying a current proportional to the currentflowing through the electrolytic cell. The proportional current circuitthen includes, in series with the other components, a fixed resistanceand a variable resistance; and the fixed resistance of this circuit isconnected also to the shunt resistor through current-sensing means whichgive a null indication when the current through the fixed resistance ofthe proportional current circuit is in a fixed proportion to the currentthrough the shunt resistor and hence to the current through theelectrolytic cell. A motor is advantageously provided to actuate thevariable resistance in the proportional current circuit. This motorpreferably is energized through a circuit actuated in response to theindications of the current-sensing means in circuit with the fixedresistance and the shunt resistor, thereby automatically to vary thevalue of the variable resistance as required to maintain the current inthe proportional current circuit at all times in fixed proportion to thecurrent through the electrolytic cell.

Ordinarily a considerable number of aluminum reduction pots or cells areconnected in series. it is advantageous to locate the bath resistanceindicating means provided by this invention at a central control point,and to provide switching means by which the terminals of any one of thecells or pots may be selectively connected to the appropriateresistances of the proportional current circuit in order to obtain ameasurement of the bath resistance of such cell. If desired, the motormeans for varying the variab e portion of the potentiometer resistancein the-proportional current circuit may also actuate a visual indicatorcalibrated to provide a direct visual indication of the cell bathresistance.

A preferred embodiment of the measuring means of this invention isdescribed herein with reference to the accompanying drawing, the singlefigure of which is a circuit diagram of the new measuring means.

A number of electrolytic cells 1, 1, .1 are connected in series (in whatis commonly called a pot line) to a primary source 2 of direct currentby a power line 3. Each cell contains a molten electrolytic bath 4 ofalumina and cryolite in a carbon-lined pot 5, the carbon lining of whichserves as the cathode of the cell. Immersed in the electrolytic bath 4is a carbon anode 6 supported by a frame which is mounted on jacks 7.Current from the source 2 flows serially through the anode 6, the bath4, and the cathode of each cell.

The total voltage across the terminals of any one of the cells is thesum of (a) the voltage drop across the fixed resistances of the cell(i.e. the anode resistance, the cathode resistance, and the resistanceof the associated conductors and connections), (b) the voltage dropacross the variable resistance of the bath between the anode and thecathode, and (c) the electrochemical decomposition potential of thealumina. In a given cell only the bath resistance varies, for the fixedresistances are determined by the construction and materials of the celland the connections to it, and the decomposition potential of alumina isa constant quantity (1.7 volts). To maintain an optimum voltage dropacross any one cell, and a substantially uniform flow of current throughit, it is therefore necessary to control the bath resistance by keepingthe anode at a substantially constant predetermined distance from thecathode. This is accomplished by lowering the anode into the bath bymeans of the jacks 7 as the lower end of the anode is consumed byoxidation.

An accurate indication of the distance through the bath between theanode and the cathode can be obtained by measuring the bath resistance.However, to do so it is first necessary to isolate a voltage drop equalto that due to bath resistance. According to the present invention thisis accomplished by compensating for the voltage drop due to fixedresistances of the cell and for the decomposition potential of alumina,and determining from the residual voltage drop the resistance of thebath.

The current through each of the cells in the pot line circuit flowsthrough an accurately calibrated shunt 8. A shunt resistor 9 ofaccurately known resistance is connected through a conventional magneticamplifier 19 in parallel with the shunt. A current i which is directlyproportional at all times to the current I through the pot lineconsequently flows through the shunt resistor.

A proportional current curcuit 11 is provided, includmg lnseries a DC.voltage source 12, the resistance element 13 of a potentiometer, anadjustable resistance 14, a variable resistance 15, and a fixedresistance 16. The fixed resistance 16 also is connected to the shuntresistor 9 through current-sensing means 17. So long as the voltage dropimpressed on the shunt resistor 9 by the magnetic amplifier is equal tothe voltage drop across the fixed resistance 16 due to current flowingin the circut 11, the current-sensing means 17 will yield a nullindication. The variable resistance 15 is controlled by a motor 18 whichis energized by a circuit 19 responsive to the fiow of current throughthe current-sensing means 17. Whenever current flows through the latter,the motor circuit is energized to actuate the motor 18 in the directionnecessary to reduce the current flow through the sensing means to zero.

Thus, an increase (or decrease) in the pot line current will cause aproportional increase (or decrease) in current through the shuntresistor 9. This will cause the voltage drop across the shunt resistor 9to exceed (or become less than) that across the fixed resistance 16. Theresulting flow of current through the current-sensing means 17 will thenenergize the motor 18 in the direction necessary to decrease (orincrease) the variable resistance 15, and thereby increase (or decrease)the current in the proportional current circuit 11 until a null responseis again obtained from the current-sensing means. By this arrangementthe current in the proportional current circuit 11 is always andautomatically maintained in fixed proportion to the current flowingthrough the pot line circuit 3, regardless of variations that occur inthe magnitude of such current.

Leads are connected from the terminals of each of the electrolytic cells1, 1' 1 to the points of a twopole multi-throw selector switch 20. Thetwo poles of the switch thus may be selectively connected to theterminals of any one of the electrolytic cells. In the circuit diagram,the switch poles are shown connected to the terminals of the first cell1.

A circuit 21 is provided to connect any one of the electrolytic cellsthrough the selector switch 20 and through a source of back E.M.F. 22across the adjustable resistance 14 and (through a potentiometer slider23) a variable portion of the potentiometer resistance element 13 of theproportional current circuit 11. The circuit 21 thus impresses acrossthese resistances the full value of the electrolytic cell voltage E,minus the amount of the back E.M.F. of the source 22. This back E.M.F.is a fixed DC. voltage equal in value to the decomposition potential ofalumina, E, (1.7 volts). The polarity of the back E.M.F. source 22 isarranged relative to the polarity of the electrolytic cell with whichthe circuit 21 is connected, so that the residual voltage E impressedacross the resistances of the proportional current circuit is equal tothe electrolytic cell voltage minus the decomposition potential ofalumina, i.e. it is 1.7 volts less than the actual cell voltage. Theresidual voltage E thus is equal to the voltage drop in the electrolyticcell due to the flow of the electrolytic cell current I through thefixed resistances and the bath resistance of the cell.

As described above, the flow of current through the proportional currentcircuit, and hence through the adjustable resistance 14 andpotentiometer resistance element 13, is automatically maintained infixed proportion to the amount of current i flowing through theelectrolytic cells. The adjustable resistance 14 can be preset to avalue such that the voltage drop through it due to the flow Of theproportional current i is equal to that portion of the voltage drop inthe electrolytic cell due to the flow of the cell current I through thefixed resistances of the cell. Then, the potentiometer slider 23 can beadjusted to a position such that the residual voltage E impressed acrossthe sum of the adjustable resistance 14 and'the variable portion of thepotentiometer resistance element 13 is just balanced by the voltage dropdue to the flow of the proportional current i through these resistances.At this position of balance, and with the adjustable resistance 14preset to the value stated above, the magnitude of the variable portionof the potentiometer resistance across which the residual voltage E isimpressed will bear a definite proportional relation to the resistanceof the electrolytic cell bath.

It is thus evident that by presetting the adjustable resistance 1 4 inthe manner above outlined, and by then current through thecurrent-sensing element.

moving the potentiometer slider 23 to balance the residual voltage Eagainst the voltage drop due to flow of proportional current i throughthe adjustable resistances 14 and the potentiometer resistance 13, anindication of the cell bath resistance can be obtained. This resistancecan be made visually observable and directly readable in magnitude bycoupling the potentiometer slider 23 to the pointer of an indicator 24which is calibrated to read directly the value of the cell bathresistance.

Preferably, in order to facilitate operation of the apparatus and renderit substantially completely automatic, a current-sensing element 25 isincluded in the circuit 21 through which the residual voltage E isimpressed across the resistances of the proportional current circuit. Amotor 26 is then provided to operate the potentiometer slider 23 and thepointer of the indicator 24. This motor 26 is actuated through anenergizing circuit 27 which is responsive to the flow, and to thedirection of flow, of Through the circuit 27 the motor is energized,whenever there is a flow of current through the sensing means 25, in adirection to move the potentiometer s ider to the point where theresidual voltage E is just balanced by the voltage drop due to flow ofthe proportional current i through the adjustable resistance 14 and thevariable portion of the potentiometer resistance. At this po nt ofbalance, the current-sensing element 25 gives a null indication anddeenergizes the motor 26.

With the circuits and apparatus described above, the actual value at anymoment of the resistance of the electro'yte bath in any one of the cellsin the pot line can be quickly and visually indicated by simp y movingthe selector switch 20 to the position where its poles are connected tothe terminals of that electrolytic cell. provided only that the settingof the adiusab e resistance 14 is such that the volta e drop due to flowof the proportional current i throu h that resistance is equal to thevoltage drop due to the flow of cell current I through the tiredresistances of the cell for which the indic tion is being ta en.Generally the fixed resistances of each of a series of identicalelectrolytic cells wi l be the same and when such is the case, noresetting of the adiustab e resistance 14 is necessary when the circuit21 is switched from one cell to another. If, however, there arediflerences in the fi ed resistances of various of the cells thendetents or o her means may be provided on the mechani m of theadjustable resistance to insure rapid setting of it at the correct valuefor the ce l to whi h connection is made through the se ector switch 20.Indeed, it is possible to interlock the selector switch and theadjustable resistance 14 so that the correct setting of the former isachieved automatically upon moving the selector switch. It is of courseapparent that once the fixed resistances of any one of the cells hasbeen determined, and the corresponding sett ng of the adjustableresistance for that cell h s been ascertained, the adjustable resistancewill always be returned to that setting whenever the bath resistance ofthat particular cell is to be determined.

With the measuring means of this invention, an accurate and rapiddetermination of the bath resistance of any electrolytic cell is a potline can be made from a convenient central location. By the provision ofsuitable remote contro-ls for the jacks 7, the anode of that cell can beadjusted from that same central location to maintain optimum length ofthe electrical path between anode and cathode.

I claim:

1. The combination with an aluminum reduction cell having a cathode andan anode which is movable relative to saidcathode immersed in anelectrolytic bath, and circuit means connecting said cell to a source ofdirect current, of means for measuring the resistance of theelectrolytic bath between the anode and cathode comprising aproportional current circuit including a potentiometer resistanceelement, an adjustable resistance and a 6 source of direct currentconnected in series, "means "for maintaining the current flowing in saidproportional current circuit in fixed proportion to the current flowingthrough said electrolytic cell, circuit means for applying a residualvoltage equal to the total electrolytic cell voltage minus thedecomposition voltage of alumina across said adjustable resistance and avariable portion of the potentiometer resistance element of saidproportional current circuit, and means for varying said variableportion of the potentiometer resistance to balance the voltage due tocurrent flowing through said variable portion and said adjustableresistance against the residual voltage applied thereto, whereby themagnitude of the variable portion of the potentiometer resistance isproportional to the resistance of the electrolytic bath when the valueof said adjustable resistance is such that the voltage drop due tocurrent flowing therethrough equals the voltage drop due to currentflowing through the fixed resistances of the electrolytic cell. p

2. The combination with an aluminum reduction cell having a cathode andan anode which is movable relative to said cathode immersed in anelectrolytic bath, and circuit means connecting said cell to a source ofdirect current, of means for measuring the: resistance of the eectrolytic bath between the anode and the cathode comprising aproportional current circuit including a potentiometer resistanceelement, an adjustable resistance and a source of direct currentconnected in series, means for maintaining the current, flowing in saidproportional current circuit in fixed proportion to the current flowingthrough said elec rolytic cell, circuit means connecting the terminalsof said cell in series with a source of back EMF. equal to thedecomposition voltage of alumina across said adjustable resistance and avariable portion of the potentiometer resistance element of saidproportional current circuit, and means for varying said variableportion of the potentiometer resistance to balance the voltage drop dueto current flowing through said variable portion and said ad'ustableresistance against the residual voltage applied thereto from theterminals of the electrolytic cell, whereby the magnitude of thevariable portion of the potentiometer resistance is proportional to theresistance of the electrolytic bath when the value of said adjustableresistance is such that the voltage drop due to current flowingtherethrough equals the voltage drop due to current flowing through thefixed resistances of the electrolytic cell,

3. The combination with an aluminum reduction cell having a cathode andan anode which is movable relative to said cathode immersed in anelectrolytic bath, and circuit means connecting said cell to a source ofdirect current, of means for measuring the resistance of theelectrolytic bath between the anode and the cathode comprising aproportional current circuit including a potentiometer resistanceelement, an adjustable resistance and a source of direct currentconnected in series, means for maintaining the current flowing in saidproportional current circuit in fixed proportion to the current flowingthrough said electrolytic cell, circuit means connecting the terminalsof said cell in series with a source of back equal to the decompositionvoltage of alumina through current-sensing means across said adjustableresistance and a variable portion of the potentiometer resistanceelemerit of said proportional current circuit, motor means for varyingsaid variable portion of the potentiometer resistance, and a motorenergizing circuit responsive to the flow of current through saidcurrent-sensing means, whereby said motor means is energized in adirection to vary the variable portion of the potentiometer resistanceto balance the voltage drop due to current flowing through said variableportion and said adjustable resistance against the voltage appliedthereto through said current-sensing means, and whereby the magnitude ofthe variable portion of the potentiometer resistance is automaticallymaintained proportional to the resistance of the electrolytic trolyticcell having a cathode and an anode which is movable relative to saidcathode immersed in an electrolytic bath, and circuit means connectingsaid cell to a source of direct current, of means for measuring theresistance of the electrolytic bath between the anode and cathodecomprising a shunt resistor connected effectively in parallel with ashunt in said electrolytic cell circuit and normally carrying a currentproportional to the current flowing through said electrolytic cell, aproportional current circuit including a potentiometer resistanceelement, an adjustable resistance, a variable resistance, and a fixedresistance connected in series with a source of direct current, thefixed resistance of said proportional current circuit being connected tosaid shunt resistor through current-sensing means, motor means forvarying said variable resistance, a motor actuating circuit energized inresponse to the flow of current through said currentsensing means forenergizing said motor means in a direction to vary said variableresistance as required to maintain the current flowing in theproportional current circuit and through the fixed resistance thereof infixed proportion to the current flowing through said shunt resistor,circuit means for applying a residual voltage equal to the totalelectrolytic cell voltage minus the decomposition voltage of aluminaacross said adjustable resistance and a variable portion of thepotentiometer resistance element of said proportional current circuit,and means for varying said variable portion of the potentiometerresistance to balance the voltage drop due to current flowing throughsaid variable portion and said adjustable resistance against theresidual voltage applied thereto, whereby the magnitude of the variableportion of the potentiometer resistance is proportional to theresistance of the electrolytic bath when the value of said adjustableresistance is such that the voltage drop due to current flowingtherethrough equals the voltage drop due to current flowing through thefixed resistances of the electrolytic cell.

6. The combination with an aluminum reduction cell having a cathode andan anode which is movable relative to said cathode immersed in anelectrolytic bath, and circuit means connecting said cell to a source ofdirect current,.of means for measuring the resistance of theelectrolytic bath between the anode and cathode comprising a shuntresistor connected efiectively in parallel with a shunt in saidelectrolytic cell circuit and normally carrying a current proportionalto the current flowing through said electrolytic cell, a proportionalcurrent circuit including a potentiometer resistance element, anadjustable resistance, a variable resistance, and a fixed resistanceconnected in series with a source of direct current, the fixedresistance of said proportional current circuit being connected to saidshunt resistor through first current-sensing means, first motor meansfor varying said variable resistance, a motor actuating circuitenergized in response to the flow of current through saidcurrent-sensing means for energizing said first motor means in adirection to vary said variable resistance as required to maintain thecurrent flowing in the proportional current circuit and through thefixed resistance thereof in fixed proportion to the current flowingthrough said shunt resistor, circuit means connecting the terminals ofsaid cell in series with a source of back E.M.F. equal to thedecomposition voltage of alumina through second current-sensing meansacross said adjustable resistance and a variable portion of thepotentiometer resistance element of said proportional current circuit,second motor means for varying said variable portion of thepotentiometer resistance, and a motor energizing circuit responsive tocurrent flow through said second current-sensing means for energizingsaid second motor means in a direction to vary the variable portion ofthe potentiometer resistance to balance the voltage drop due to currentflowing through said variable portion and said adjustable resistanceagainst the voltage applied thereto through said second currentsensingmeans, whereby the magnitude of the variable portion of thepotentiometer resistance is automatically maintained proportional to theresistance of the electrolytic bath when said adjustable resistance hasbeen preset to a value such that the voltage drop due to current flowingtherethrough equals the voltage drop due to current flowing through thefixed resistances of the electrolytic cell.

7. The combination according to claim 3, in which a plurality ofaluminum reduction cells are connected in series, including a selectorswitch for connecting the terminals of any one of said cells selectivelythrough the source of back and the current-sensing means across thevariable portion of the potentiometer resistance and the adjustableresistance.

8. The combination with a plurality of aluminum reduction cells eachhaving a cathode and an anode which is movable relative to said cathodeimmersed in an electrolytic bath, and circuit means connecting saidcells in series with a source of direct current, of means for measuringthe resistance of the electrolytic bath between the anode and cathode ofany one of said cells comprising a proportional current circuitincluding a potentiometer resistance element, an adjustable resistanceand a source of direct current connected in series, means formaintaining the current flowing in said proportional current circuit infixed proportion to the current flowing through said series ofelectrolytic cells, circuit means including a selector switch forconnecting the terminals of any one of said cells through a source ofback E.M.F. equal to the decomposition voltage of alumina across saidadjustable resistance and a variable portion of the potentiometerresistance element of said proportional current circuit, and means forvarying said variable portion of the potentiometer resistance to balancethe voltage drop due to current flowing through said variable portionand said adjustable resistance against the voltage applied thereto fromthe terminals of an electrolytic cell, whereby the magnitude of thevariable portion of the potentiometer resistance is proportional to theresistance of the electrolytic bath in that cell to which it isconnected through said selector switch means when said adjustableresistance is preset to a value such that the voltage drop due tocurrent flowing therethrough equals the voltage drop due to currentflowing through the fixed resistances of said electrolytic cell.

References Cited in the file of this patent UNITED STATES PATENTS2,545,411 Perret-Bit Mar. 13, 1951 2,545,412 Ferret-Bit Mar. 13, 19512,545,413 Ferret-Bit Mar. 13, 1951 2,731,412 Ferrand Jan. 17, 1956

1. THE COMBINATION WITH AN ALUMINUM REDUCTION CELL HAVING A CATHODE ANDAN ANODE WHICH IS MOVABLE RELATIVE TO SAID CATHODE IMMERSED IN ANELECTROLYTIC BATH, AND CIRCUIT MEANS CONNECTING SAID CELL TO A SOURCE OFDIRECT CURRENT, OF MEANS FOR MEASURING THE RESISTANCE OF THEELECTROLYTIC BATH BETWEEN THE ANODE AND CATHODE COMPRISING APROPORTIONAL CURRENT CIRCUIT INCLUDING A POTENTIOMETER RESISTANCEELEMENT, AN ADJUSTABLE RESISTANCE AND A SOURCE OF DIRECT CURRENTCONNECTED IN SERIES, MEANS FOR MAINTAINING THE CURRENT FLOWING SAIDPORPORTIONAL CURRENT CIRCUIT IN FIXED PROPORTION TO THE CURRENT FLOWINGTHROUGHT SAID ELECTROLYTIC CELL, CIRCUIT MEANS FOR APPLYING A RESIDUALVOLTAGE EQUAL TO THE TOTAL ELECTROLYTIC CELL VOLTAGE MINUS THEDECOMPOSTION VOLTAGE OF ALUMINA ACROSS SAID ADJUSTABLE RESISTANCE ANDVARIABLE PORTION OF THE POTENTIOMENER RESISTANCE ELEMENT OF SAIDPROPORTIONAL CURRENT CIRCUIT, AND MEANS FOR VARYING SAID VARIABLEPORTION OF THE POTENTIOMENTER RESISTANCE TO BALANCE THE VOLTAGE DUE TOCURRENT FLOWING THROUGH SAID VARIABLE PORTION AND SAID ADJUSTABLERESISTANCE AGAINST THE RESIDUAL VOLTAGE APPLIED THERETO, WHEREBY THEMAGNITUDE OF THE VARIABLE PORTION OF THE POTENTIOMETER RESISTANCE ISPROPORTIONAL TO THE RESISTANCE OF THE ELECTROLYTIC BATH WHEN THE VALUEOF SAID ADJUSTABLE RESISTANCE IS SUCH THAT THE VOLTAGE DROP DUE TOCURRENT FLOWING THERETHROUGH EQUALS THE VOLTAGE DROP DUE TO CURRENTFLOWING THROUGH THE FIXED RESISTANCE OF THE ELECTROLYTIC CELL.